U.S. patent application number 09/818713 was filed with the patent office on 2002-04-25 for laser interferometer displacement measuring system, exposure apparatus, and elecron beam lithography apparatus.
Invention is credited to Hosaka, Sumio, Isshiki, Fumio, Sugaya, Masakazu, Suzuki, Tatsundo, Yomaoka, Masahiro.
Application Number | 20020048026 09/818713 |
Document ID | / |
Family ID | 18605298 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020048026 |
Kind Code |
A1 |
Isshiki, Fumio ; et
al. |
April 25, 2002 |
Laser interferometer displacement measuring system, exposure
apparatus, and elecron beam lithography apparatus
Abstract
An absolute accuracy in the range from .+-.2 nm to .+-.1 nm for
a displacement measurement value is provided by a laser
interferometer displacement measuring system. A fluctuating error
component that appears corresponding to the wave cycle of laser
light is detected and subtracted from the measurement value while a
stage is moving, thereby providing a high accuracy.
Inventors: |
Isshiki, Fumio; (Kokubunji,
JP) ; Sugaya, Masakazu; (Kawasaki, JP) ;
Suzuki, Tatsundo; (Musashimurayama, JP) ; Yomaoka,
Masahiro; (Hachioji, JP) ; Hosaka, Sumio;
(Hinode, JP) |
Correspondence
Address: |
ANTONELLI TERRY STOUT AND KRAUS
SUITE 1800
1300 NORTH SEVENTEENTH STREET
ARLINGTON
VA
22209
|
Family ID: |
18605298 |
Appl. No.: |
09/818713 |
Filed: |
March 28, 2001 |
Current U.S.
Class: |
356/498 |
Current CPC
Class: |
H01J 37/3045 20130101;
H01J 2237/20292 20130101; G03F 7/70775 20130101 |
Class at
Publication: |
356/498 |
International
Class: |
G01B 009/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 28, 2000 |
JP |
2000-89556 |
Claims
What is claimed is:
1. A laser interferometer displacement measuring system comprising
a displacement measurement mechanism making use of laser
interference, and corrector means for adding a correction value to
or subtracting the correction value from a measurement value of
said displacement measurement mechanism, wherein said corrector
means uses a cyclic correction value having a cycle corresponding
to a wave cycle of laser light.
2. A laser interferometer displacement measuring system comprising
a displacement measuring mechanism making use of laser
interference, and corrector means for adding a correction value to
or subtracting the correction value from a measurement value of
said displacement measurement mechanism, wherein said corrector
means has storage means for storing a cyclic correction value
having a cycle corresponding to a wave cycle of laser light, and
the correction value is read out of said storage means in
accordance with said measurement value and is added to or
subtracted from said measurement value.
3. A laser interferometer displacement measuring system, comprising
a laser light source, an interferometer for dividing laser light of
wavelength .lambda. emitted from said laser light source into a
reference path beam and a measurement path beam to interfere said
reference path beam with the measurement path beam having been
reflected from a subject body, a light detector for detecting the
light subjected to the interference in said interferometer, and
measurement value output means for converting a detection signal of
said light detector into a measurement value to output the
resulting value, in which a displacement of the subject body causes
an n-fold variation in length of an optical path between said
interferometer and the subject body, said laser interferometer
displacement measuring system, further comprising corrector means
for adding a correction value to or subtracting the correction
value from the measurement value of said measurement value output
means, wherein with said measurement value being employed as a
variable, said corrector means uses, as the correction value, a
cyclic function having a cycle of .lambda./n or a sum of a
plurality of cyclic functions having said cycle of .lambda./n as a
fundamental cycle.
4. The laser interferometer displacement measuring system according
to claim 3, wherein the plurality of cyclic functions having said
cycle of .lambda./n as a fundamental cycle are the cyclic function
having a cycle of .lambda./n and harmonic cyclic functions
thereof.
5. The laser interferometer displacement measuring system according
to any one of claims 1 to 4, further comprising means for
performing feedback control so as to carry out tracking adjustment
of a phase and amplitude of the correction value.
6. A laser interferometer displacement measuring system comprising
a displacement measurement mechanism making use of laser
interference, and corrector means for adding a correction value to
or subtracting the correction value from a measurement value of
said displacement measurement mechanism, wherein said corrector
means prepares a plurality of types of cyclic functions having a
cycle corresponding to a wave cycle of laser light, and assigns
weights to each of the cyclic functions to allow the resulting
cyclic functions to be added to or subtracted from said measurement
value.
7. The laser interferometer displacement measuring system according
to any one of claims 1 to 6, wherein averaging unit means capable
of averaging over time is provided after said corrector means.
8. A laser interferometer displacement measuring system comprising
a displacement measurement mechanism making use of laser
interference, error signal component generating means for
eliminating a constant speed component and an acceleration
component from a measurement value of said displacement measurement
mechanism and generating an error signal component, storage means
for storing the error signal component generated from said error
signal component generating means corresponding to said measurement
value, and means for allowing the error signal component stored in
said storage means to be added to or subtracted from the
measurement value of said displacement measurement mechanism as a
correction value.
9. A laser interferometer displacement measuring system, comprising
a laser light source, an interferometer for dividing laser light of
wavelength .lambda. emitted from said laser light source into a
reference path beam and a measurement path beam to interfere said
reference path beam with the measurement path beam having been
reflected from a subject body, a light detector for detecting the
light subjected to the interference in said interferometer, and
measurement value output means for converting a detection signal of
said light detector into a measurement value to output the
resulting value, in which a displacement of the subject body causes
an n-fold variation in length of an optical path between said
interferometer and the subject body, said laser interferometer
displacement measuring system, further comprising corrector means
for adding a correction value to or subtracting the correction
value from the measurement value of said measurement value output
means, wherein said corrector means comprises means for storing or
calculating, with said measurement value being employed as a
variable, a cyclic function having a cycle of .lambda./n or said
cyclic function having a cycle of .lambda./n and harmonic cyclic
functions thereof, error signal component generating means for
eliminating a constant speed component and an acceleration
component from a measurement value of said displacement measurement
mechanism and generating an error signal component, adjustment
means for adjusting an amplitude and a phase of said cyclic
function so that said cyclic function having a cycle of .lambda./n
or a sum function of said cyclic function having a cycle of
.lambda./n and harmonic cyclic functions thereof fits to said error
signal component, and means for allowing a function value of said
cyclic function having a cycle of .lambda./n or a function value of
said sum function of said cyclic function having a cycle of
.lambda./n and harmonic cyclic functions thereof to be added to or
subtracted from said measurement value.
10. An apparatus comprising a stage for placing thereon and moving
a sample or a subject work, drive means for driving said stage, and
a laser interferometer displacement measuring system for measuring
a position of said stage, wherein as said laser interferometer
displacement measuring system, the laser interferometer
displacement measuring system according to any one of claims 1 to 9
is employed.
11. A laser interferometer displacement measuring system,
comprising a laser light source, an interferometer for dividing
laser light emitted from said laser light source into a reference
path beam and a measurement path beam to interfere said reference
path beam with the measurement path beam having been reflected from
a subject body, a light detector for detecting the light subjected
to the interference in said interferometer, phase detector means
for obtaining a phase value of a detection signal of said light
detector, accumulator means for accumulating a variation in said
phase value, and corrector means for allowing a correction value to
be added to or subtracted from an accumulated value of said
accumulator means, said laser interferometer displacement measuring
system wherein said corrector means uses, as the correction value,
a cyclic value with said phase value being employed as a
variable.
12. A laser interferometer displacement measuring system,
comprising a laser light source, an interferometer for dividing
laser light of wavelength .lambda. emitted from said laser light
source into a reference path beam and a measurement path beam to
interfere said reference path beam with the measurement path beam
having been reflected from a subject body, a light detector for
detecting the light subjected to the interference in said
interferometer, phase detector means for obtaining a phase value of
a detection signal of said light detector, and accumulator means
for accumulating a variation in said phase value, said laser
interferometer displacement measuring system further comprising
corrector means for allowing a correction value to be added to or
subtracted from an accumulated value of said accumulator means,
wherein said corrector means uses a cyclic correction value having
a cycle of wavelength .lambda. of laser light with said accumulated
value being employed as a variable.
13. A laser interferometer displacement measuring system,
comprising a laser light source, an interferometer for dividing
laser light of wavelength .lambda. emitted from said laser light
source into a reference path beam and a measurement path beam to
interfere said reference path beam with the measurement path beam
having been reflected from a subject body, a light detector for
detecting the light subjected to the interference in said
interferometer, phase detector means for obtaining a phase value of
a detection signal of said light detector, and accumulator means
for accumulating a variation in said phase value, said laser
interferometer displacement measuring system further comprising
corrector means for allowing a correction value to be added to or
subtracted from an accumulated value of said accumulator means and
generating means for generating said correction value, wherein said
generating means detects synchronously a signal component having a
frequency of a cycle of wavelength .lambda. contained in said
time-dependent accumulated value.
14. A laser interferometer displacement measuring system,
comprising a laser light source, an interferometer for dividing
laser light of wavelength .lambda. emitted from said laser light
source into a reference path beam and a measurement path beam to
interfere said reference path beam with the measurement path beam
having been reflected from a subject body, a light detector for
detecting the light subjected to the interference in said
interferometer, phase detector means for obtaining a phase value of
a detection signal of said light detector, and accumulator means
for accumulating a variation in said phase value, said laser
interferometer displacement measuring system further comprising
corrector means for allowing a correction value to be added to or
subtracted from an accumulated value of said accumulator means and
generating means for generating said correction value, wherein said
generating means for generating a correction value detects a signal
component having a frequency of a cycle of wavelength .lambda.
contained in the time-dependent accumulated value corrected by said
corrector means and carries out feedback control so as to minimize
the signal component.
15. The laser interferometer displacement measuring system
according to any one of claims 11 to 14, wherein in an optical
system thereof, the measurement path beam travels twice or more
between the reflector and the interferometer.
16. A laser interferometer displacement measuring system,
comprising a laser light source, an interferometer for dividing
laser light of wavelength .lambda. emitted from said laser light
source into a reference path beam and a measurement path beam to
interfere said reference path beam with the measurement path beam
having been reflected from a subject body, and a light detector for
detecting the light subjected to the interference in said
interferometer, in which a variation in length of an optical path
of the measurement path beam caused by a movement of the subject
body is n (a natural number) times a displacement of the subject
body, said laser interferometer displacement measuring system
further comprising means for suppressing a relative peak intensity,
with respect to a baseline of a frequency spectrum, of a peak of
frequency component f=Nv/.lambda. (N is a natural number of 1 to 2
n and not equal to n) of a signal indicative of the amount of
received light, the signal being generated in said light detector
due to a movement of said subject body at speed v.
17. A laser interferometer displacement measuring system,
comprising a laser light source, an interferometer for dividing
laser light of wavelength .lambda. emitted from said laser light
source into a reference path beam and a measurement path beam to
interfere said reference path beam with the measurement path beam
having been reflected from a subject body, and a light detector for
detecting the light subjected to the interference in said
interferometer, in which a variation in length of an optical path
of the measurement path beam caused by a movement of the subject
body is n (a natural number) times a displacement of the subject
body, said laser interferometer displacement measuring system
wherein a relative peak intensity, with respect to a baseline of a
frequency spectrum, of a peak of frequency component f=Nv/.lambda.
(N is a natural number of 1 to 2 n and not equal to n) of a signal
indicative of the amount of received light, the signal being
generated in said light detector due to a movement of said subject
body at speed v, is suppressed for output relative to said signal
indicative of the amount of received light in a frequency spectrum
of a signal of a measurement value.
18. A laser interferometer displacement measuring system,
comprising a laser light source, an interferometer for dividing
laser light emitted from said laser light source into a reference
path beam and a measurement path beam to interfere said reference
path beam with the measurement path beam having been reflected from
a subject body, a light detector for detecting the light subjected
to the interference in said interferometer, phase detector means
for obtaining a phase value of a detection signal of said light
detector, accumulator means for accumulating a variation in said
phase value, and storage means for storing a correction value for
allowing the correction value to be added to or subtracted from the
accumulated value of said accumulator means, said laser
interferometer displacement measuring system wherein a cyclic value
with said phase value being employed as a variable is used as said
correction value.
19. An exposure apparatus comprising a stage for placing a sample
thereon, a light source for emitting light of wavelength 160 nm or
less to radiate said sample with the light, means for focusing the
light from said light source onto said sample, and a laser
interferometer displacement measuring system for measuring a
displacement of said stage, said exposure apparatus further
comprising corrector means for making correction of said wave cycle
of laser to a measuring board of said laser interferometer
displacement measuring system.
20. An electron beam lithography apparatus comprising a stage for
placing a sample thereon, an electron beam source for radiating
said sample with an electron beam, beam shaping means, interposed
between said stage and said electron beam source, for shaping said
electron beam into a shaped beam, and a laser interferometer
displacement measuring system for measuring a displacement of said
stage, said electron beam lithography apparatus further comprising
corrector means for generating a signal, in phase with a wave cycle
of laser light, of a displacement measurement value outputted from
said laser interferometer displacement measuring system to add the
resulting signal to said displacement measurement value.
21. An electron beam lithography apparatus comprising a stage for
placing a sample thereon, an electron beam source for radiating
said sample with an electron beam, deflection control means for
deflecting said electron beam, and a laser interferometer
displacement measuring system for measuring a displacement of said
stage, said electron beam lithography apparatus further comprising
means, provided with a function for carrying out drawing by
radiating the electron beam while said stage is continuously
moving, for suppressing a signal, in phase with a wave cycle of
laser light, of a displacement measurement value outputted from
said laser interferometer displacement measuring system to add the
resulting signal to said deflection control means.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to displacement measurement
techniques, instrumentation techniques, evaluation techniques,
precision patterning techniques, fine patterning techniques,
semiconductor patterning techniques, and master mask patterning
techniques. More particularly, the present invention relates to a
displacement measurement technique which requires accuracy of the
order of nanometer.
[0003] 2. Description of the Related Art
[0004] For example, a laser interferometer displacement measuring
system is often used as high accuracy displacement measurement
means for controlling such as a stepper, employed in the
photolithography process for fabricating semiconductor devices, and
for controlling X-Y stages for use such as in precision machining
equipment. A nominal value of resolution is 0.3 nm for the
displacement measurement system which provides the most accurate
displacement measurement and has been developed particularly for
stepping control.
[0005] Concerning the provision of increased accuracy, even a
general displacement measurement technique that does not employ the
laser interferometer displacement measuring technique but employs
noise processing by averaging is disclosed in Japanese Patent
Laid-Open Publication No. Hei 7-306034 in relation to the
non-contact displacement measurement. Optical measurement employing
an optical interferometer is disclosed in Japanese Patent Laid-Open
Publication No. Hei 9-178567 in relation not to position but to
wavelength measurement.
[0006] However, in many cases, even the current laser
interferometer displacement measuring system having a nominal value
of resolution of 0.3 nm actually provides only an absolute accuracy
of the order of .+-.2 nm for displacement measurement. The
resolution and the absolute accuracy are essentially different from
each other. An interferometer displacement measuring system may
apparently have an accuracy of 0.3 nm in the range of about 10 nm,
but in some cases, a gradual undulation may be generally found with
the magnitude reaching 3nm or more in the range of 100 to 300 nm.
These problems were not made clear until a high-speed real-time
displacement measurement approach was developed to thereby make it
possible to measure the displacement of a moving object with high
accuracy at a frequency greater than that of mechanical
vibrations.
[0007] In general, to provide an increased accuracy, the noise
processing by averaging over time is performed as mentioned above
in order to improve the accuracy (i.e., relative accuracy) of
stability of measurement values under a standstill condition of the
object. However, with a recent increasing demand for increased
accuracy, the absolute accuracy of measurement values has become
necessary. In the course of study to the present invention, such a
problem has become clear that the prior-art noise processing by
averaging cannot provide a sufficient absolute accuracy.
[0008] In view of the aforementioned problems, it is therefore the
object of the present invention to provide a high-accuracy
interferometer displacement measuring system which provides an
absolute accuracy in the range of .+-.2 nm to .+-.1 nm or less for
a displacement measurement value using interference of laser
light.
SUMMARY OF THE INVENTION
[0009] In consideration of the fact that the interference of light
itself, which is the principle of laser interferometry, causes an
error, the present invention is adapted to eliminate errors,
concerning absolute accuracy, which cannot be eliminated only by
averaging over time. Approaches to increased accuracy that focus
attention on such a cause of error have never discussed before.
More specifically, a correction value corresponding to the laser
wave cycle of a displacement is added to a displacement output of
the laser interferometer displacement measuring system, thereby
correcting the distortion error in the interferometer displacement
measuring system.
[0010] Upon measurement of a continuously moving object as a
measurement target, the laser interferometer displacement measuring
system according to the present invention stores and corrects, as a
measurement error caused by the interference effect, an oscillatory
component that appears in the cycle consistent with the frequency
of laser light, thereby implementing an increased accuracy. Even
such a correction method for allowing a relatively simple
sinusoidal wave to be added to or subtracted from a measurement
value can reduce the range of error of absolute position about
.+-.2 nm to within .+-.1 nm, thereby making it possible to provide
an increased accuracy.
[0011] That is, a laser interferometer displacement measuring
system according to the present invention is characterized by
comprising a displacement measurement mechanism making use of laser
interference, and corrector means for adding a correction value to
or subtracting the correction value from a measurement value of the
displacement measurement mechanism. The corrector means uses a
cyclic correction value having a cycle corresponding to a wave
cycle of laser light.
[0012] Furthermore, a laser interferometer displacement measuring
system according to the present invention is characterized by
comprising a displacement measuring mechanism making use of laser
interference, and corrector means for adding a correction value to
or subtracting the correction value from a measurement value of the
displacement measurement mechanism. The corrector means has storage
means for storing a cyclic correction value having a cycle
corresponding to a wave cycle of laser light, and the correction
value is read out of the storage means in accordance with the
measurement value and is added to or subtracted from the
measurement value. It is possible to employ a rewritable memory as
the storage means.
[0013] Furthermore, a laser interferometer displacement measuring
system according to the present invention comprises a laser light
source, an interferometer for dividing laser light of wavelength
.lambda. emitted from the laser light source into a reference path
beam and a measurement path beam to interfere the reference path
beam with the measurement path beam having been reflected from a
subject body, a light detector for detecting the light subjected to
the interference in the interferometer, and measurement value
output means for converting a detection signal of the light
detector into a measurement value to output the resulting value. In
the system, a displacement of the subject body causes an n-fold
variation in length of an optical path between the interferometer
and the subject body. The laser interferometer displacement
measuring system is characterized by further comprising corrector
means for adding a correction value to or subtracting the
correction value from the measurement value of the measurement
value output means. The system is also characterized in that, with
the measurement value being employed as a variable, the corrector
means uses, as the correction value, a cyclic function having a
cycle of .lambda./n or a sum of a plurality of cyclic functions
having the cycle of .lambda./n as a fundamental cycle. The
plurality of cyclic functions having the cycle of .lambda./n as a
fundamental cycle can be the cyclic function having a cycle of
.lambda./n and harmonic cyclic functions thereof.
[0014] The aforementioned laser interferometer displacement
measuring system according to the present invention can comprise
means for performing feedback control so as to carry out tracking
adjustment of a phase and amplitude of the correction value.
[0015] A laser interferometer displacement measuring system
according to the present invention is characterized by comprising a
displacement measurement mechanism making use of laser
interference, and corrector means for adding a correction value to
or subtracting the correction value from a measurement value of the
displacement measurement mechanism. The corrector means prepares a
plurality of types of cyclic functions having a cycle corresponding
to a wave cycle of laser light, and assigns weights to each of the
cyclic functions to allow the resulting cyclic functions to be
added to or subtracted from the measurement value.
[0016] It is also possible to employ mathematically orthogonal
cyclic functions as the plurality of types of correction value
cyclic functions. For example, as the mathematically orthogonal
cyclic function, it is possible to use the sinusoidal (sin) and
cosine (cos) functions and a group of harmonics of these cyclic
functions. In addition, as the plurality of types of correction
value cyclic functions, it is possible to use a triangular wave
function and a group of orthogonal harmonic cyclic functions of the
triangular wave. Incidentally, the plurality of types of correction
value cyclic functions do not have necessarily to be mathematically
orthogonal cyclic functions.
[0017] To use functions that are orthogonal to each other as the
plurality of types of cyclic functions, the system can be provided
with calculation means for calculating the magnitude of the
component of each cyclic function in the cyclic error contained in
a measurement value by integrating each cyclic function
individually with the measurement value to perform the weighting by
means of the output of the calculation means.
[0018] Furthermore, the system can be provided with phase shift
means for shifting the phase of the plurality of types of cyclic
functions at the same time to perform feedback control on the
amount of shift provided by the phase shift means.
[0019] It is preferable to make the feedback time constant of the
amount of shift provided by the phase shift means shorter than the
feedback time constant of other amplitudes. It is preferable to
provide the system with means for enabling the feedback control
only when the subject body is moving at a given speed or
greater.
[0020] Furthermore, it is possible to dispose averaging means
capable of averaging over time after the aforementioned corrector
means. It is also possible to configure the system to bypass the
averaging processing provided by the averaging unit means for
output.
[0021] Furthermore, a laser interferometer displacement measuring
system according to the present invention is characterized by
comprising a displacement measurement mechanism making use of laser
interference, and error signal component generating means for
eliminating a constant speed component and an acceleration
component from a measurement value of the displacement measurement
mechanism and generating an error signal component. The system
further comprises storage means for storing the error signal
component generated from the error signal component generating
means corresponding to the measurement value, and means for
allowing the error signal component stored in the storage means to
be added to or subtracted from the measurement value of the
displacement measurement mechanism as a correction value.
[0022] Furthermore, a laser interferometer displacement measuring
system according to the present invention comprises a laser light
source, an interferometer for dividing laser light of wavelength
.lambda. emitted from the laser light source into a reference path
beam and a measurement path beam to interfere the reference path
beam with the measurement path beam having been reflected from a
subject body, a light detector for detecting the light subjected to
the interference in the interferometer, and measurement value
output means for converting a detection signal of the light
detector into a measurement value to output the resulting value. In
the system, a displacement of the subject body causes an n-fold
variation in length of an optical path between the interferometer
and the subject body. The laser interferometer displacement
measuring system is characterized by further comprising corrector
means for adding a correction value to or subtracting the
correction value from the measurement value of the measurement
value output means. The corrector means comprises means for storing
or calculating, with the measurement value being employed as a
variable, a cyclic function having a cycle of .lambda./n or the
cyclic function having a cycle of .lambda./n and harmonic cyclic
functions thereof, and error signal component generating means for
eliminating a constant speed component and an acceleration
component from a measurement value of the displacement measurement
mechanism and generating an error signal component. The corrector
means further comprises adjustment means for adjusting an amplitude
and a phase of the cyclic function so that the cyclic function
having a cycle of .lambda./n or a sum function of the cyclic
function having a cycle of .lambda./n and harmonic cyclic functions
thereof fits to the error signal component, and means for allowing
a function value of the cyclic function having a cycle of
.lambda./n or a function value of the sum function of the cyclic
function having a cycle of .lambda./n and harmonic cyclic functions
thereof to be added to or subtracted from the measurement
value.
[0023] The system can be configured such that correction processing
is carried out by means of hardware in a real time manner or
implemented by software means in conjunction with a mechanism for
calculating correction values.
[0024] A laser interferometer displacement measuring system
according to the present invention makes it possible to drive a
displacement measurement subject at a given speed upon activation
or initialization of the system to acquire correction data at that
time. In addition, a laser interferometer displacement measuring
system according to the present invention may be adapted to be
combined with a stage control device to set a correction value of a
correction table for use in laser displacement measurement at the
time of initial operation or control of the stage.
[0025] The measurement value correcting means or means for
implementing the method for correcting measurement values can be
integrated on a measuring board (counter board or axis board) for
laser interferometry.
[0026] A laser interferometer displacement measuring system
according to the present invention can be mounted on a single-axis
stage, a multi-axis stage, or an X-Y stage.
[0027] An apparatus according to the present invention comprises a
stage for placing thereon and moving a sample or a subject work,
drive means for driving the stage, and a laser interferometer
displacement measuring system for measuring a position of the
stage. The apparatus is characterized in that, as the laser
interferometer displacement measuring system, the aforementioned
laser interferometer displacement measuring system is employed.
Examples of those apparatuses include an electron beam lithography
system, a stepper for fabricating semiconductors (an exposure
apparatus), a fine patterning system, metal machining equipment,
ceramic machining equipment, mask pattern transfer equipment, mask
patterning equipment, an electron-beam scanning microscope with a
displacement measurement function, a transmission electron
microscope with a displacement measurement function, and
non-contact shape measurement equipment.
[0028] A laser interferometer displacement measuring system
according to the present invention comprises a light detector for
detecting the light subjected to interference, phase detector means
for detecting a phase from a detection signal of the light
detector, accumulator means for accumulating variations in phase
obtained from the phase detector means, correction value generating
means for generating a correction value from an accumulated value
provided by the accumulator means or the phase value, and corrector
means for allowing the correction value, generated by the
correction value generating means, to be added to the accumulated
value or the phase value. The correction value generating means
generates a cyclic correction value of wavelength .lambda. of laser
light with the accumulated value or the phase value being employed
as a variable and eliminates a signal component produced in phase
with the wave cycle of the laser light.
[0029] The correction value generating means generates a correction
value with an accumulated value, not a phase value, or variations
in phase value being employed as a variable, thereby generating a
cyclic correction value at a cycle of .lambda. independent of n.
The correction value is generated in the cycle of .lambda., thereby
extracting the error of a plurality of cyclic components
corresponding to each harmonic component of 1 to 2 n harmonic waves
of wavelength .lambda..
[0030] The laser interferometer displacement measuring system
according to the present invention comprises means for suppressing
a relative peak intensity, with respect to a baseline of a
frequency spectrum, of a peak of frequency component f=Nv/.lambda.
(N is a natural number of 1 to 2 n and not equal to n) of a signal
generated in the light detector due to a movement of the subject
body at speed v. This allows for eliminating those frequency
components to provide increased accuracy for the laser
interferometer displacement measuring system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] In the accompanying drawings:
[0032] FIG. 1 is a view showing the principle of a correction
method according to the present invention;
[0033] FIG. 2 is a view showing the overall configuration of a high
accuracy displacement measuring system according to the present
invention, including a laser interferometer displacement measuring
system and a driving system;
[0034] FIG. 3 is a view showing time-dependent variations in
measurement value when the displacement of a stage moving at a
constant speed (5 mm per second) is measured by a laser
interferometer displacement measuring system without any
correction;
[0035] FIG. 4 is a view showing time-dependent variations in
measurement value when the displacement of a stage moving at a
constant speed (40 mm per second) is measured by a laser
interferometer displacement measuring system without any
correction;
[0036] FIG. 5 is a view showing the flow of signals in an exemplary
configuration of distortion error corrector means according to the
present invention;
[0037] FIG. 6 is a view showing an exemplary configuration of
distortion error corrector means according to the present
invention, in which the portion required for high speeds is
constructed by hardware, showing the circuit configuration of the
hardware portion and a feedback path;
[0038] FIG. 7 is a view showing an exemplary relatively simple
configuration which allows for an automatic tracking correction in
a distortion error correction process according to the present
invention;
[0039] FIG. 8 is a view showing the examples of two cyclic
functions (shown by a solid and dotted line, respectively),
orthogonal in phase to each other, for use in automatic tracking
distortion error correction processing according to the present
invention;
[0040] FIG. 9 is a view showing the examples of two cyclic
functions (shown by a solid and dotted line, respectively),
orthogonal in phase to each other, for use in automatic tracking
distortion error correction processing according to the present
invention;
[0041] FIG. 10 is a view showing an exemplary configuration of
automatic tracking distortion error correction processing according
to the present invention, in which employed are two cyclic
functions orthogonal in phase to each other;
[0042] FIG. 11 is a view showing an exemplary configuration of
automatic tracking distortion error correction processing according
to the present invention, in which employed are three or more
cyclic functions orthogonal mathematically to each other;
[0043] FIG. 12 is a view showing an example of a group of cyclic
functions for use in the exemplary configuration of FIG. 11;
[0044] FIG. 13 is a view showing an exemplary configuration of
automatic tracking distortion error correction processing means
according to the present invention, in which the amplitude control
of a plurality of cyclic functions orthogonal to each other is
combined with the phase tracking control;
[0045] FIG. 14 is a view showing an example of a group of cyclic
functions for use in the exemplary configuration of FIG. 13;
[0046] FIG. 15 is a view showing the order of connection in the
signal processing for the combination of distortion corrector means
and averaging means according to the present invention;
[0047] FIG. 16 is a view showing the frequency spectrum of a
coordinate displacement signal with a stage being at a standstill
before processing is performed by averaging means;
[0048] FIG. 17 is a view showing an exemplary configuration
comprising means which is referenced as an input to averaging
processing only when a certain traveling speed of a stage is
exceeded in the automatic tracking correction processing of
distortion error;
[0049] FIG. 18 is a view showing the overall exemplary
configuration in which a laser interferometer displacement
measuring system according to the present invention is configured
using phase measurement means (phasemeter);
[0050] FIG. 19 is a view showing the overall exemplary
configuration of a laser interferometer displacement measuring
system for implementing a correction method in which a correction
value is generated employing a phase value as a variable to add the
resulting correction value to the phase value;
[0051] FIG. 20 is a view showing the overall exemplary
configuration of a laser interferometer displacement measuring
system for implementing a correction method in which a correction
value is generated employing a phase value as a variable to add the
resulting correction value to an accumulated value;
[0052] FIG. 21 is a view showing a laser interferometer
displacement measuring system with a four-fold measurement path
(n=4) in which an optical error is occurring in the optical
system;
[0053] FIG. 22 is a view showing a laser interferometer
displacement measuring system with an eight-fold path (n=8) in
which an optical error is occurring in the optical system;
[0054] FIG. 23 is a view showing an actual example of a correction
result provided when the sum of cyclic functions of 1 to 2 n
harmonic waves is employed as a correction value;
[0055] FIG. 24 is a view showing an exemplary configuration of an
exposure system employing a laser interferometer displacement
measuring system according to the present invention;
[0056] FIG. 25 is a view showing an exemplary configuration of an
electron beam lithography apparatus employing a laser
interferometer displacement measuring system according to the
present invention;
[0057] FIG. 26 is a view showing the entire exemplary configuration
of a laser interferometer displacement measuring system according
to the present invention, which employs a method for performing
automatic tracking of and updating a correction value to provide
increased accuracy;
[0058] FIG. 27 is a view showing the entire exemplary configuration
of a laser interferometer displacement measuring system according
to the present invention, which employs a method for performing an
automatic feedback of a correction value to provide increased
accuracy; and
[0059] FIG. 28 are views showing examples of frequency spectrum of
a measurement value; FIG. 28A showing an example without an
accuracy improvement method according to the present invention,
FIG. 28B showing the other example with the method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0060] Now, the present invention will be explained below with
reference to the accompanying drawings in accordance with the
embodiments.
[0061] [Embodiment 1]
[0062] Basic Configuration of a Laser Interferometer Displacement
Measuring System with a Distortion Correction Function
[0063] The overall exemplary configuration of a laser
interferometer displacement measuring system according to the
present invention is shown in FIG. 2. In the figure, shown is an
actual example in which laser displacement measurement is employed
for measuring a single-axis stage with accuracy and performing
feedback control. The single-axis stage has a motor or the like as
a stage driving power source 7 and detects the distance of a
movable stage table 6 using variations in position of a reflector 8
on the stage table.
[0064] A laser power supply 1 drives a gas laser light source 2 to
generate laser light 3, which is in turn reflected on a beam bender
4 and then introduced into an interferometer 5. The optical path is
divided into two paths inside the interferometer 5. One of the
optical paths is the measurement path in which the light reaches
the reflector 8 on the stage table 6 and is then reflected thereon
to return to the interferometer 5. The other optical path is the
reference path in which the light is reflected inside the
interferometer 5. This example employs a four-fold optical path (in
which light travels twice between the interferometer 5 and the
reflector 8) for laser displacement measurement. The light beams of
the measurement path and the reference path are mixed in the
interferometer 5. Mixing the two light beams causes interference to
occur. The light having caused the interference is launched from
the interferometer 5 and then detected by a light detector 9. The
light detector 9 detects the light and converts the detected amount
of light into an electrical signal. A measuring board 10 converts
the resulting electrical signal into a coordinate value and then
outputs the resulting value as a measurement value 13. Means for
increasing displacement output value accuracy 11 corrects the
measurement value 13 to output a measurement value 23 with
increased accuracy. A personal computer 12 for collecting data and
performing control captures the value to perform feedback to the
position of the stage and a correction mechanism. Incidentally, the
measuring board 10 is generally called a counter board or an axis
board, but is herein consistently referred to as the measuring
board.
[0065] The present invention relates particularly to a correction
method and means, employed in the means 11 for increasing
displacement output value accuracy. In this embodiment, this is
hereinafter described as independent processing means as shown in
FIG. 2. This is because this function is implemented in the form of
electrical signal processing and can be implemented by either
hardware or software. Thus, it is made possible to implement this
function by incorporating the means 11 as hardware into the
measuring board 10 or by incorporating the means 11 as software
into the personal computer 12. It is also possible to use
independent hardware to implement an independent configuration as
shown in FIG. 2.
[0066] The gas laser light source 2 employed in this embodiment is
a He--Ne gas laser for emitting laser light at a wavelength of 633
nm. A high-frequency electromagnetic wave is applied to the gas for
excitation. Incidentally, a vacuum chamber can be employed to seal
the interferometer 5 and the subsequent portions (the
interferometer 5, the stage table 6, and the reflector 8) therein
to provide the measurement path in a vacuum, thereby making it
possible to prevent an error in measurement caused by variations in
refractivity due to a fluctuation of air or a change in humidity of
air. Furthermore, to maintain the accuracy of measurement, a
multi-layered coating is applied to the components such as the beam
bender 4, the interferometer 5, and a transparent window which are
attached to the wall of the vacuum chamber in order to prevent
unnecessary multiple reflections.
[0067] The laser interferometer displacement measuring system
configured as such makes it possible to provide measurement values
or coordinate outputs at a resolution of 0.3 nm and a high sampling
rate of 10 MHz. A specific signal example is shown in FIG. 3.
[0068] FIG. 3A shows variations in measurement value, measured
using the system configuration of FIG. 2 at a resolution of 0.3 nm
and a 10 MHz sampling rate while the single-axis stage is moving at
a constant speed (5 mm per second). The movement at the constant
speed of the single-axis stage causes the measurement values to
apparently move linearly with respect to time. However, it would be
found that the measurement values, when magnified, are actually not
on a straight line. FIG. 3B shows a signal obtained by subtracting
a linear component and a slight acceleration component (a parabolic
component) from the signal of FIG. 3A with the scale of the
vertical axis being enlarged. Incidentally, the original signal is
the same as that of FIG. 3A. The horizontal axis has been changed
from the time to the position (length) scale, however, the data is
taken from the same time-interval. As can be seen from FIG. 3B, the
measurement value fluctuates in the range of about .+-.2 nm and
varies cyclically. The fluctuation has four cycles for about 630
nm.
[0069] FIG. 4 shows similar data that has been obtained at a
different traveling speed of the stage. FIG. 4A shows
time-dependent variations in measurement value provided when the
single-axis stage moves at a speed of 40 mm per second. FIG. 4B
shows a signal obtained by subtracting a linear component and a
parabolic component from the signal of FIG. 4A. Other points are
the same as those FIGS. 3A and B. As in FIG. 3, the stage looks as
if it moves linearly in FIG. 4A. However, in FIG. 4B where the
linear and parabolic components have been eliminated from the
measurement values, it can be found that the measurement values
fluctuate with an amplitude of about .+-.2 nm in a cyclic manner.
The fluctuation has four cycles in the range of about 630 nm, which
are generally the same as those of FIG. 3. The wavelength of the
He--Ne laser employed as the laser light source has a wavelength of
633 nm and the aforementioned cycles are exactly consistent with
that of the alternating light and dark pattern caused by the
interference of the laser light. This shows that the cycle is an
error in measurement caused by variations in quantity of laser
light at a high frequency in the light detector 8.
[0070] Due to this error, the measurement of a displacement of 70
nm or greater would provide an absolute accuracy in the range of an
error of about .+-.2 nm. Thus, even with such a laser
interferometer displacement measuring system with a nominal value
of resolution of 0.3 nm, the value of absolute accuracy would be
measured being deteriorated several times in some cases as
described above.
[0071] To correct this deterioration, the waveforms obtained as
shown in FIGS. 3B and 4B may be pre-stored as a correction value to
subtract the correction value from subsequently obtained
measurement values. This makes it possible to increase accuracy.
The cycle of fluctuation corresponds exactly to that of the laser
wavelength. Accordingly, based on the value obtained by
measurement, a cyclic value may be generated corresponding to the
wavelength cycle of laser light and used as a correction value. The
correction may be either carried out giving a high priority to a
real-time property (the property of real-time processing) by means
of hardware or may be implemented by software means in conjunction
with a mechanism for calculating correction values (shown in
Embodiments 2 to 5).
[0072] A method for processing a signal to implement the correction
is shown in FIG. 5. FIG. 5 shows an example of the means 11 for
increasing displacement output value accuracy, by which the
measurement value 13 outputted from the measuring board 10 is
captured to output a measurement value 23 with increased accuracy.
In accordance with the captured measurement value 13, a correcting
value 20 is read from a memory device 19 and corrected by a
adder/subtractor 22, then being outputted as the measurement value
23 with increased accuracy. Designated as 25 is a parabolic
component extracting filter unit for outputting a phase shifting
value 17.
[0073] The correcting value 20 is generated as described below.
First, a phase adder 16 adds the phase shifting value 17 to the
measurement value 13 and then outputs the resulting value as a
table reference address 18. In accordance with the table reference
address 18, the memory device 19 outputs the correcting value 20
stored therein. The memory device 19 has data stored therein, which
is to be outputted as the correcting value 20 and which provides a
cyclic value corresponding to the wavelength cycle of laser light.
The value can be set to a given one by the means 21 for
manipulating values stored in the memory. It is desirable that the
table reference address 18 be cyclic in accordance with the cycle
of the laser wavelength. In this regard, only the upper bits equal
to or greater than the wavelength cycle can be ignored when the
measurement value of laser interferometry employs those outputted
digitally with two to the power of N being adopted as the
wavelength cycle. More specific values to be stored in the table
are the waveforms (or one cycle of the waveform) shown in FIGS. 3B
and 4B, which may be stored as they are. Besides this, a sinusoidal
(sin) waveform having the same cycle can be employed to allow the
correction to provide the same effect of increasing accuracy.
[0074] FIG. 1 shows the effect of increasing accuracy according to
the present invention. FIG. 1 shows a case where the waveform of
fluctuation of FIG. 3B is subtracted by the oscillation component
of a single sinusoidal wave as a correction value, thereby
providing increased accuracy. In this case where the original
waveform has been sufficiently tracked, it is found that accuracy
can be increased about three times with respect to the original
accuracy to be in the range of about .+-.0.6 nm. It is possible to
further improve this accuracy by optimizing the waveform of the
correction value to be employed for the subtraction. This method
will be described in Embodiments 3 to 5. In the Embodiments 2 and
3, shown is the configuration of a system for fitting a sinusoidal
wave by automatic tracking, the system being provided with an
increased accuracy shown in FIG. 1.
[0075] Incidentally, in the configuration of this system, the
memory device 19 is not inevitably necessary. Values corresponding
to the table may be calculated on the spot. In many cases, however,
simultaneous use of a memory device may generally facilitate
configuration of the system. This provides such flexibility that
processing can be performed at high speeds with no operation time
being required and any given values can be set without employing
any operational equations. The phase adder 16 and the phase
shifting value 17 are also not essential but provide increased
flexibility and an advantage of not having to rewrite all values of
the table in the memory when it is desired to shift only the phase
of the correction value.
[0076] Incidentally, take the operation of subtracting the linear
and acceleration components from FIG. 3A to yield FIG. 3B, and the
operation of converting FIG. 4A into FIG. 4B. These operations can
be implemented specifically by fitting the quadratic function
expressed in the form of equation 1 shown below to the measurement
values by the least square method in a statistical manner and then
by subtracting the average displacement value of the resulting
fitting.
[0077] [Equation 1]
y(x)=ax.sup.2+bx+c
[0078] where "x" is the horizontal axis and "y" is the vertical
axis.
[0079] This fitting can be carried out by determining constants a,
b, and c with respect to "yi" and "xi" in accordance with equation
2 employing a determinant in a statistical manner. That is, 1 ( a b
c ) = ( X4 X3 X2 X3 X2 X1 X2 X1 N ) - 1 ( X2Y X Y Y1 ) X2Y = i = 1
N ( x i 2 y i ) , X Y = i = 1 N ( x i y i ) , Y1 = i = 1 N y i , X1
= i = 1 N x i , X2 = i = 1 N x i 2 , X3 = i = 1 N x i 3 , X4 = i =
1 N x i 4 [ Equation 2 ]
[0080] where "yi" is the measurement value either in the range of
about four cycles before and after the desired center of correction
or in the range of about six cycles before or after the point, and
"xi" is the coordinate of each point.
[0081] The determined a, b, and c are substituted for the quadratic
equation again to determine "y" with respect to the point "x", the
error of which is to be determined. A parabolic component
extracting filter 25 performs this processing. The operation can be
implemented by subtracting the "y" determined as such from
"yi".
[0082] Summarizing the aforementioned procedure, first, the
measurement values 13 that are outputted continuously from the
measuring board 10 are fitted by a quadratic function to subtract a
fitted average displacement from an actual measurement value,
thereby determining the fluctuation error shown in FIGS. 3B and 4B.
With this fluctuation error being employed as a correction value,
the correction value is subtracted from subsequent measurements,
thereby making it possible to provide a measurement value which has
a fluctuation error cancelled out and is increased in accuracy.
[0083] FIG. 6 is a view showing an exemplary circuit for
specifically implementing this by using a hardware-wise circuit
configuration. The measurement value 13 from the laser
interferometer displacement measuring system is divided into upper
bits 14 corresponding to the wave cycle number of the laser and
lower bits 15 indicative of the position of decomposing one
wavelength cycle. Among these lower bits, only the signal of the
lower bits is extracted and added to the phase shifting value 17 at
the phase adder 16. Thereafter, the phase value 18 after shifting
is inputted as the address of a dual port RAM (Random Access
Memory) 24. Incidentally, the dual port RAM is employed as the
memory device here. With the presetting values 21 being pre-stored
in the dual port RAM 24, the correcting value 20 is read in
accordance with the phase value 18 after shifting. This correction
value is added in the adder/subtractor 22 to the original
measurement value 13 (the position coordinate signal produced by
combining the upper bits 14 corresponding to the wave cycle number
with the lower bits 15 indicative of the position of decomposing
one wavelength cycle), thereby providing the displacement value 23
after distortion correction. The correction value for addition may
be normally sufficiently about 8 bits, and the upper bit portion
insufficient for the number of bits (about 32 bits) of the
measurement value is extended by 8 bits for addition.
[0084] As described above, the measurement value 13 of the laser
interferometer displacement measuring system is typically
configured to count and output the wavelength cycle number in order
to simplify the configuration of the hardware. This makes it
possible to divide the measurement value 13 into the bus of the
upper bits 14 corresponding to the wave cycle number of the laser
and the lower bits 15 indicative of the position of decomposing one
wavelength cycle. In the aforementioned embodiment, a cyclic
correction is implemented with a simple configuration by making use
of this feature.
[0085] The correction value to be stored in the dual port RAM 24 is
the same as the operation for subtracting the aforementioned
parabolic component from the measurement value 13. In this figure,
this calculation process is to be carried out using the personal
computer 12. The procedure and equations to be processed are the
same as those described in the foregoing.
[0086] Incidentally, as described above, the configuration of the
measurement value corrector means for storing correction values on
the rewritable memory allows a given cyclic function to be set as a
correction value, thereby providing a high flexibility. Thus, by
improving the calculation method of the correction value in the
correction value calculation means to be combined with the
measurement value correction means, the configuration provides an
advantage of allowing accuracy to be improved possibly to a level
close to the limit that can be provided by this correction
method.
[0087] [Embodiment 2]
[0088] Configuration of Automatic Phase Tracking Means for
Increasing Accuracy
[0089] FIG. 7 shows an exemplary configuration of automatic
amplitude tracking distortion correction means which performs
feedback control on the phase of cyclic function of a correction
value to correct the distortion error of a wave cycle.
[0090] The measurement value 13 obtained by the measuring board is
captured and then subtracted by the value that has passed through
the parabolic component extracting filter 25, thereby providing a
distortion error signal 26. This calculation method is the same as
that described in Embodiment 1 using the determinant. In addition,
the phase shifting value 17 is added to the measurement value 13 in
the phase adder 16 to generate the table reference address 18. This
value is employed as the input to the memory device 19 for a
generating cyclic function value and to a cyclic orthogonal
function table 36 having a phase orthogonal to the cyclic function.
Incidentally, the memory device 19 may be a ROM (Read Only Memory)
having fixed values stored therein. In addition, the memory device
19 and the cyclic orthogonal function table 36 are not limited to
the memory device but more generally may be replaced with a
function calculation mechanism as far as the mechanism can generate
cyclic values. However, the shape of the function to be generated
has to be suitable for correction.
[0091] A cyclic function value 30 generated as described above and
a cyclic orthogonal function value 37 are multiplied by the
distortion error signal 26 at multipliers 31, 38 to obtain a
correction strength, thereby providing a correlation ratio 32 and a
correlation ratio 39 for cyclic orthogonal function. These values
are averaged with respect to time in a time-averaging filter 33 and
an accumulator 40, thereby making it possible to provide an average
correlation strength of each signal component.
[0092] An example of appropriate pairs of cyclic functions to be
generated at the memory device 19 and the cyclic orthogonal
function table 36 is shown in FIGS. 8 and 9. The solid line of FIG.
8 represents a sinusoidal (sin) wave and the dotted line represents
a cosine (cos) wave. When a four-fold optical path (in which light
travels twice between the interferometer and the reflector) for
laser displacement measurement is used, such a function that takes
on a cyclic value at each .lambda./4 may be employed (where
.lambda. is the wavelength of the laser).For example, the
sinusoidal wave is selected as the value to be set to the memory
device 19, while the cosine wave is selected as the value to be set
to the cyclic orthogonal function table 36. When the sinusoidal
wave generated by the memory device 19 lags in phase behind the
distortion error signal 26, these settings provide a positive
correlation with the cosine wave component orthogonal in phase to
the sinusoidal wave, thereby providing the correlation ratio 39 for
cyclic orthogonal function with a longer time for taking on a
positive value than for taking on a negative value. The correlation
ratio 39 for cyclic orthogonal function is added to the accumulator
40, gradually increasing the phase shifting value 17 to be
outputted from the accumulator. Accordingly, the table reference
address 18 increases to thereby restore the delayed phase. On the
contrary, when the sinusoidal wave leads in phase the distortion
error signal 26, a reverse operation is carried out to cause a
decrease in the address.
[0093] This feedback allows the phase of the sinusoidal wave
outputted from the memory device 19 to be always consistent with
the phase of the fundamental wave component of the distortion error
signal 26. Under this condition, oscillatory component of the
sinusoidal wave or a correction value is outputted from a
multiplier 35 in accordance with an averaged correlation intensity
34 provided by the time-averaging filter 33. The correcting value
20 is subtracted from the original measurement value 13 in the
adder/subtractor 22 to provide the displacement value 23 after
distortion correction.
[0094] Incidentally, as can be seen from FIGS. 3B and 4B, the
waveform of the distortion signal appearing in the distortion error
signal 26 takes on the shape of a triangular wave rather than a
sinusoidal wave. By using a pair of triangular waves (each shown by
a solid and dotted line in FIG. 9) having phases different from
each other by 90.degree. in place of the sinusoidal and cosine
waves shown in FIG. 8, it is possible to carry out correction more
efficiently. Thus, a flexibility is given to the selection of a
cyclic function.
[0095] Incidentally, the automatic phase tracking control can also
be implemented with the configuration shown in FIG. 6 or a
combination of hardware and software means. The phase adder 16
prepared in front of the memory device 19 can be used to replace
the aforementioned phase feedback processing by software-wise
processing on the personal computer 12 to thereby implement the
distortion correction processing shown in FIG. 7 with the
configuration of FIG. 6. In this case, although no mechanism is
available for adjusting amplitudes, this mechanism can be
implemented by choosing several cyclic function values, which have
different amplitudes and are prepared in the dual port RAM 24, in
conjunction with the presetting values 21.
[0096] [Embodiment 3]
[0097] Configuration of Automatic Amplitude Tracking Distortion
Corrector Means (1)
[0098] FIG. 10 shows an exemplary configuration of automatic
amplitude tracking distortion corrector means for correcting the
distortion error of a measurement value by automatically
controlling the amplitude of two cyclic functions.
[0099] The measurement value 13 provided by the measuring board is
captured and then subtracted by the value that has passed through
the parabolic component extracting filter 25 to thereby provide the
distortion error signal 26. This calculation method is the same as
that described in Embodiment 2. On the other hand, the measurement
value 13 is employed as it is to be inputted to the memory device
19 having the two cyclic functions therein and to the cyclic
orthogonal function table 36. Each of the generated cyclic function
value 30 and the cyclic orthogonal function value 37 is multiplied
by the cyclic orthogonal function table 36 at each of the
multipliers 31, 38, thereby providing the correlation of each
cyclic function component included in the error signal. These
correlations represent the correlation ratio 32 and the correlation
ratio 39 for cyclic orthogonal function. Each of these values is
allowed to pass though the time-averaging filter 33, thereby
providing the averaged correlation intensity 34 and an averaged
correlation intensity 41 for the cyclic orthogonal function, which
represent the magnitude of each cyclic function component. Each
cyclic function value is added to each other in proportion to the
magnitude of each component. Suppose a combination of a sinusoidal
and a cosine wave is used as the cyclic function. In this case, a
sinusoidal wave having any phase can be expressed with a linear
superposition of these two waveforms. Thus, the phase feedback
described in Embodiment 2 with reference to FIG. 7 is not always
necessary and the same effect can be provided only by such
amplitude control.
[0100] The amplitude of the cyclic function value 30 and the cyclic
orthogonal function value 37 is varied in accordance with the
signal intensity of each component or the averaged correlation
intensity 34 and the averaged correlation intensity 41 for cyclic
orthogonal function, thereby making the resulting amplitude
consistent with the magnitude of each cyclic signal component
included in the distortion error signal 26. This is carried out
with multipliers 35, 42. As described above, a cyclic function
value 43 and an orthogonal cyclic function value 44 are provided as
the output of the multipliers. A sum correction value 46 that is
obtained by adding each signal component of the cyclic functions in
a full adder 45 is subtracted from the original measurement value
13 at the adder/subtractor 22, thereby providing the displacement
value 23 after distortion correction with cyclic distortion being
eliminated.
[0101] As described above, it is possible to configure automatic
distortion error corrector means for tracking phases without
employing a feedback portion for tracking phases. Incidentally, the
two cyclic functions used here, or the sinusoidal and cosine waves,
are the same as those shown in FIG. 8 (by a solid and dotted
line).
[0102] [Embodiment 4]
[0103] Configuration of Automatic Amplitude Tracking Distortion
Corrector Means (2)
[0104] FIG. 11 shows an exemplary configuration of automatic
amplitude tracking distortion corrector means which is configured
by expanding the configuration described in Embodiment 3 to
automatically control the amplitude of three or more cyclic
functions in order to provide further increased accuracy.
[0105] Here, the sinusoidal and cosine waves and a group of
harmonics of these cyclic functions shown in FIG. 12 are used in
place of the two sinusoidal wave and cosine waves, shown in FIG. 8,
used in Embodiment 3.
[0106] As shown in FIG. 11, the automatic amplitude tracking
distortion corrector means is provided with a number of mechanisms,
arranged in parallel to each other corresponding to the number of
cyclic functions used, for generating the two cyclic functions
shown in FIG. 10. Instead of the memory device 19 and the cyclic
orthogonal function table 36, this configuration employs the memory
device 19 and a plurality of cyclic orthogonal function tables 47,
48. Although FIG. 11 shows only three mechanisms, a required number
of mechanisms corresponding to the number of cyclic functions used
are arranged in parallel to each other. The configuration
subsequent to the full adder 45 is the same as that of FIG. 10.
[0107] The sinusoidal and cosine waves and harmonics of these
cyclic functions are mathematically orthogonal to each other. This
provides an advantage of allowing the intensity of amplitude
corresponding to each component to be independently calculated only
by the calculation of correlation using an accumulator.
[0108] Using a group of orthogonal functions as described above
would make it possible to generate a given cyclic function only by
the assignment of weights to and addition of each cyclic function.
The orthogonality makes it possible to separately determine the
intensity of the component corresponding to each function by means
of an accumulator. Furthermore, the value of three or more cyclic
functions including harmonics thereof can be added to thereby track
a distortion error signal having finer irregularities and reproduce
the shape of the distortion error signal with a higher fidelity.
This provides an advantage of reducing a residual or a distortion
error by subtraction to thereby increase accuracy.
[0109] [Embodiment 5]
[0110] Configuration of Combined Corrector Means of Automatic
Amplitude Tracking and Phase Tracking
[0111] FIG. 13 shows an exemplary configuration of combined
automatic distortion corrector means, in which automatic amplitude
tracking control and automatic phase tracking control are combined
with each other using a plurality of orthogonal cyclic
functions.
[0112] In this configuration, the phase tracking feedback portion
shown in FIG. 7 and the amplitude control of the plurality of
cyclic functions shown in FIG. 11 are combined with each other. The
flow of signal and function of each portion are the same in each of
the portions bearing the same reference numbers of the embodiments
2 and 4. With such a configuration, it is possible to select a
given group of orthogonal cyclic functions other than sinusoidal
and cosine waves as the group of cyclic functions to be used.
[0113] An example of a cyclic function to be used in this
embodiment includes a group of orthogonal harmonics of a
triangular-wave cyclic function. The triangular wave having been
shown in FIG. 9 (by a solid line) is expressed in the form of the
following equation 3. That is,
[0114] [Equation 3]
y=Tri(x)
[0115] where x is the horizontal axis and y is the vertical
axis.
[0116] With the group of orthogonal harmonics of a triangular-wave
cyclic function being expressed in the form of equation 4 (where n
is a natural number), each of the harmonic cyclic functions having
n=1 to 8 is expressed in the form of equation 5.
[0117] [Equation 4]
y=orthoTri(nx)
[0118] [Equation 5]
orthoTri(1x)=Tri(1x)
orthoTri(2x)=Tri(2x)
orthoTri(3x)=Tri(3x)+1/9Tri(1x)
orthoTri(4x)=Tri(4x)
orthoTri(5x)=Tri(5x)-1/25Tri(1x)
orthoTri(6x)=Tri(6x)+1/9Tri(2x)
orthoTri(7x)=Tri(7x)+1/49Tri(1x)
orthoTri(8x)=Tri(8x)
[0119] These functions are specifically shown in FIG. 14. All of
these functions look a triangular-wave harmonic cyclic function
with the vertex of each triangular wave being slightly shifted
vertically due to their orthogonality. Each of these orthogonal
functions is selected so as to provide zero to the integration of
the product of one function and another over one cycle in the
interval from 0 to .lambda./4.
[0120] The cyclic orthogonal function table 36 may generate a
triangular-wave function of n=1, shown by a dotted line in FIG. 14,
or a cosine-wave function of n1, shown by a dotted line in FIG.
12.
[0121] The method shown in Embodiment 4 requires both harmonic
cyclic functions of sinusoidal and cosine waves to provide phase
flexibility to increase accuracy, thereby requiring an arrangement
for generating a number of cyclic functions. In contrast to this,
from the viewpoint of phase, the method shown in this embodiment
simplifies the configuration only by the addition of one feedback
system to reduce the number of cyclic functions by one-half.
Furthermore, the method does not need to be bound to a function
such as a sinusoidal wave that can provide a given shift in phase
by superposition, thus making it possible to employ from the
beginning a triangular wave close to a distortion error wave as a
cyclic function. This makes it possible to reproduce efficiently
the waveform of a distortion error signal including a number of
triangular wave components by means of a relatively small number of
cyclic function value generating means (the memory device 19 or the
cyclic orthogonal function tables 47, 48). Thus, this provides an
advantage of simplifying the configuration and reducing costs.
[0122] Incidentally, in this automatic phase tracking distortion
correction, the feedback control can be stabilized by setting
feedback time to a value slightly shorter than the time constant of
the time-averaging filter 33 for use in amplitude control. Here,
the feedback time for feedback adjustment to provide phase
consistency is determined by the integral time constant of the
accumulator 40.
[0123] [Embodiment 6]
[0124] Configuration Combined with Noise Reduction Averaging
Means
[0125] FIG. 15 shows an exemplary configuration having a
combination of distortion corrector means 27 described in
Embodiments 1 to 5 and averaging unit (averaging means) 28 for
processing noise, allowing for measurement at higher resolution
than the minimum resolution of the original measurement value
13.
[0126] The averaging means 28 receives and then allows the
displacement value 23 after distortion correction, provided by the
distortion corrector means 27, to be averaged for output by the
method of moving averages in the digital averaging unit
incorporated therein over the same average time as the excitation
cycle of lasing. More specifically, the hardware configuration we
have used provides the displacement value 23 after distortion
correction in the form of coordinates data of 32 bits in a 0.1
.mu.s cycle. The value is averaged by the method of moving averages
by means of hardware over an average time of 14.6 .mu.s, which is
the same as the excitation cycle of lasing.
[0127] FIG. 16 shows the frequency spectrum of a signal of the
displacement value 23 after distortion correction with the stage at
a standstill. As can be seen in the figure, in addition to fine
noise around 1 MHz, a high sharp noise component corresponding to
the excitation frequency of the gas laser appears frequently near
68 kHz. Accordingly, for example, a filter having such a frequency
characteristic as shown in equation 6 below is used for averaging.
2 sin ( 2 f ) 2 f [ Equation 6 ]
[0128] where .DELTA..tau. is the time constant of the filter and f
is the frequency.
[0129] Specifically, a moving average filter may be employed as the
filter having the characteristic shown in the above equation. The
average time can be determined to be 14.6 .mu.s corresponding to
the cycle of 68 kHz, thereby making it possible to eliminate the
excitation noise of the aforementioned gas laser. Incidentally, the
aforementioned average time corresponds to 2 .DELTA..tau..
[0130] The averaging filter produces a response delay and phase
shift and therefore the distortion correction processing described
in Embodiments 1 to 5 has to be performed before this averaging
processing. For this reason, to use the averaging process in
combination with the distortion correction processing, the
distortion correction processing has to be performed before the
averaging processing as shown in FIG. 16.
[0131] Furthermore, suppose the system is configured such that only
the measurement value 29 with increased accuracy or a final output
can be detected externally as coordinates output value. In this
case, since the phase shift occurs as described above by allowing
the value to pass through the averaging means 28, the system is
desirably configured to allow the value to bypass the averaging
means 28 at the time of calculating a correction value. Similarly,
at the time of calculating a correction value, the reference of the
output value that has been corrected with the previous correction
value would make the calculation of the correction value further
complicated. To prevent this, it would provide an increased
flexibility that the distortion corrector means 27 itself can
control whether or not the correction value is to be added.
[0132] Incidentally, such a prediction of distortion error
(calculation of a correction value) would work properly only when
the stage is moving smoothly in a continuous manner at a given
constant speed (more specifically at 2 mm per second or greater).
Use of a continuous coordinates output value measured at some other
time would cause the error contained in the value to be increased
due to the mechanical vibration, acceleration, or deceleration of
the stage. Suppose the stage moves at a given constant speed or
less in the automatic tracking correction processing of a
distortion error shown in Embodiments 2 to 5. When the stage moves
at a lower speed, it is desirable to provide means for the
time-averaging filters 33, 40 shown in FIGS. 7, 10, 11, and 13
either to lock the average value without referencing the input
value or to reference on a priority basis the input value as the
input of the averaging processing only when the stage moves at a
given speed or greater. For this purpose, for example, as shown in
FIG. 17 corresponding to FIG. 7, the system may be configured to
differentiate the measurement value 13 with a derivation filter 50
and issue an enable signal for activating the time-averaging
filters 33, 40 only when the value (the traveling speed of the
stage) exceeds a certain given threshold value.
[0133] In addition, from that viewpoint, in the system employing
the means for increasing displacement output value accuracy
according to the present invention, it is desirable for the stage
or a measurement subject to be able to move smoothly in a
continuous manner at an apparently constant speed (more
specifically, at a speed variation ratio of 0.05% or less) at the
time of setting or resetting the correction value. It is thus
desirable to incorporate the movement of the stage and the
measurement target subject and the measurement processing for this
purpose into the process (procedure) to be performed at the time of
initialization and correction. That is, the process (procedure) can
be implemented only in the system having a laser interferometer
displacement measuring system integrated with driving means. On the
other hand, smooth movement of the stage would not necessarily
require a constant speed thereof. A nearly constant speed would
make it easier to carry out an error calculation by the subtraction
of a parabolic component with accuracy and allow for a linear
fitting instead of a parabolic fitting, thus facilitating
calculation.
[0134] As shown in the foregoing, the method according to the
present invention allows a laser interferometer displacement
measuring system to be provided with a reduced cyclic measurement
error caused by the interference effect of laser, thereby
implementing a higher absolute measurement accuracy than before.
Incidentally, in the foregoing, the present invention has been
explained with reference to an example with a four-fold optical
path (in which light travels twice between the interferometer 5 and
the reflector 8) for laser displacement measurement, employing
.lambda./4 as the fundamental cycle of a cyclic function (where
.lambda. is the wavelength of the laser).
[0135] With a two-fold optical path (in which light travels once
between the interferometer 5 and the reflector 8) for laser
displacement measurement, the fundamental cycle of a cyclic
function is .lambda./2. In general, the fundamental cycle of a
cyclic function is .lambda./n with an n-fold optical path (in which
light travels n times between the interferometer 5 and the
reflector 8) for laser displacement measurement.
[0136] According to the present invention, it is possible to
provide improved measurement accuracy to devices employing a laser
interferometer displacement measuring system. Thus, the present
invention is applicable to mechanisms or devices, which require
among other things a considerably high absolute accuracy, such as a
single-axis stage, an X-Y stage, a multi-axis stage, an electron
beam lithography apparatus, a stepper for fabricating
semiconductors, a fine patterning equipment, precision patterning
equipment, metal machining equipment, ceramic machining equipment,
mask pattern transfer equipment, mask patterning equipment, an
electron-beam scanning microscope with a displacement measurement
function, a transmission electron microscope, and non-contact shape
measurement equipment.
[0137] The laser interferometer displacement measuring system has
originally a high absolute accuracy in the range of a longer
distance than the wavelength of laser light, and in conjunction
with the present invention, the absolute accuracy can be improved
in the scale of the frequency or less. This makes it possible to
provide high accuracy devices with improved machining accuracy.
[0138] Suppose the distortion correction processing according to
the present invention is performed in accordance with a measurement
result provided by the laser interferometer displacement measuring
system upon control of movement of the stage or a measurement
subject. In this case, performing the distortion correction
processing on feedback control would make it possible to prevent
the occurrence of unnecessary lasing by a cyclic measurement
distortion component in the laser displacement measurement, thus
providing an advantage of facilitating readily the stability of
control.
[0139] [Embodiment 7]
[0140] Configuration of Laser Interferometer Displacement Measuring
System Employing Phase Detection
[0141] FIGS. 18 to 22 show the exemplary configuration of a laser
interferometer displacement measuring system employing a phase
meter equipped with a phase tracking circuit (PLL or Phase-Locked
Loop). The exemplary configurations of FIGS. 18 to 21 show a laser
interferometer displacement measuring system in which the
displacement of a subject body having the reflector 8 causes a
four-fold change in length of the optical path to the optical path
length of the measurement path beam between the interferometer and
the subject body. FIG. 22 shows an optical system in which a
displacement of the subject body causes an eight-fold change in
length of the optical path. Incidentally, the optical system with
either the four-fold or eight-fold optical path has the same
configuration of the signal processing system subsequent to the
light detector 9.
[0142] Now, described below is the principle of the aforementioned
laser interferometer displacement measuring system according to the
present invention. First, referring to FIG. 21, the configuration
of the optical system is described.
[0143] The laser light 3 emitted from the gas laser light source 2
impinges upon a polarizing beam splitter 51a. The laser light is
linearly polarized and provided with a polarization at an angle of
45 with respect to the polarizing beam splitter 51a. The laser
light is split into two beams: one beam (reference path beam) to be
reflected on the polarizing beam splitter 51a towards a
retroreflector 52a and the other beam (measurement path beam) to be
transmitted as it is towards a polarizing beam splitter 51b. The
reference path beam reflected towards the retroreflector 52a is
reflected twice in the retroreflector to return to the optical path
on the upper side of the polarizing beam splitter 51a, then being
reflected to the right at the reflecting plane of the polarizing
beam splitter 51a to impinge upon the light detector 9. On the
other hand, the measurement path beam transmitted towards the
polarizing beam splitter 51b passes through the subsequent
polarizing beam splitter 51b as it is and then a quarter wave plate
53, being polarized circularly to reach the reflector 8. Upon being
reflected on the reflector 8 and then passing again through the
quarter wave plate 53, the measurement path beam, circularly
polarized, is rotated by 90.degree. with respect to the original
polarization. The laser light with a polarization rotated by
90.degree. is reflected on the reflecting surface of the polarizing
beam splitter 51b towards a retroreflector 52b. Then, this light is
reflected twice in the retroreflector 52b to return to the optical
path on the upper side of the polarizing beam splitter 51b. The
light, still having the polarization rotated by 90.degree., is
reflected to the left at the reflecting plane of the polarizing
beam splitter 51b and then passes through the quarter wave plate 53
to reach again the reflector 8 being circularly polarized. Then,
upon being reflected on the reflector 8 to pass again through the
quarter wave plate 53, this light is rotated again by 90.degree. to
be provided with the original polarization and then passes through
the polarizing beam splitter 51b as it is. Then, the light passes
through the polarizing beam splitter 51a as it is to reach the
light detector 9. At this time, the light is mixed with the
reference path beam that has first reached there via the
retroreflector 52a to optically interfere with each other. A light
beam provided with an alternating light and dark pattern by the
interference of light is collected by and detected in the light
detector 9. Incidentally, it is to be understood here that the
light detector 9 also has the function of an optical detector. It
is also to be understood that a detected signal of the optical
detector or a signal indicative of an analog quantity proportional
to the amount of light received is referred to as a signal
indicative of the amount of light received.
[0144] The path between the quarter wave plate and the reflector 8
is referred to as a measurement path 70. Since the measurement path
beam travels twice along the measurement path 70, a displacement in
the reflector 8 causes a four-fold variation in the length of the
optical path, through which the measurement path beam passes, with
respect to the displacement of the reflector 8. This causes a
displacement of .lambda./4 in the reflector 8 to produce a phase
difference between the measurement and reference path beams to
thereby allow interference of laser light to occur, resulting in a
blink of the light detected by the light detector 9. The count of
the blinks makes it possible to measure a variation in length of
the measurement optical path. For light of wavelength 633 nm, one
blink corresponds to a displacement of 158 nm. To measure a
displacement with an accuracy of 1 nm, it is necessary to measure
this light blink accurately at a higher resolution than one
wavelength. For this purpose, phase detector means such as a phase
meter is generally provided using synchronization detection such as
with a phase-locked loop (PLL). By this signal processing, the
phase variation of the measurement and reference path beams is
accurately measured.
[0145] Now, the configuration subsequent to the light detector 9 is
explained with reference to FIG. 18.
[0146] Variations in intensity of the blinks detected by the light
detector 9 are inputted into phase detector means 55 as a
photodetector detection signal 54. As described above, the phase
detector means 55 measures accurately the phase difference between
the measurement and reference path beams in accordance with the
variation in intensity of interference light corresponding to the
light blinks in order to determine a phase value 56. The amount of
variations in the phase value 56 is accumulated in accumulator
means 57 to determine an accumulated value 58 corresponding to the
displacement of the reflector. The accumulated value 58, close to
the measurement value 13, has an error caused by the optical system
or circuitry. To configure a high accuracy laser interferometer
displacement measuring system, it is necessary to eliminate errors
contained in the accumulated value 58 by correction. The laser
interferometer displacement measuring system according to the
present invention is provided with a correction function for
reducing these errors. As shown in Embodiment 1, the aforementioned
accumulated value 58 often includes an error mainly composed of
wavelength .lambda. of the laser light or harmonics thereof. In
general, in a laser interferometer displacement measuring system,
the aforementioned accumulated value 58 is provided in the form of
the sum of an integral multiple of wavelength and the dividing
ratio of one wavelength. This is because the laser interferometer
displacement measuring system counts a distance by the number of
wavelengths. Therefore, it is generally easy to obtain cyclic
components of wavelength .lambda. of the laser light from the
aforementioned accumulated value. The cyclic components of
wavelength .lambda. of the laser light are detected synchronously
from the accumulated value 58. When a vibration component is found
which has a frequency that never appears under a normal condition,
it can be determined mechanically that the component is the error
of the laser interferometer displacement measuring system. A
correction value generator means 61 performs the foregoing. The
approach described in Embodiments 1 to 6 can be used for this
purpose. A correction value 60 obtained by the correction value
generator means 61 is added to the aforementioned accumulated value
58 by means of a correction value adder 59, thereby providing the
measurement value 13 with increased accuracy.
[0147] Incidentally, as an alternative configuration to that of
FIG. 18, there is available another configuration, as shown in FIG.
19, in which the correction value adder 59 is inserted in between
the phase detector means 55 and an accumulator (accumulator means)
57 to generate the correction value 60 with the phase value 56
being employed as a variable and then the resulting value is added
to the phase value 56 itself. On the other hand, FIG. 20 shows a
configuration in which the correction value adder 59 is arranged
behind the accumulator means 57 to add the resulting value to the
accumulated value 58. However, in these configurations, the phase
value 56 is employed as a variable to generate the correction value
60, thereby limiting to less than .lambda./4 the cycle of the
correction value to be added. The configuration shown in FIG. 18
provides an advantage of being available for an error of a
decreased cycle.
[0148] Now, the cause of occurrence of measurement error will be
explained below referring back to FIG. 21.
[0149] The configuration of the optical system of the laser
interferometer displacement measuring system shown in FIG. 21
usually employs lens components that are applied with
non-reflective coating to prevent unnecessary reflection of light.
However, an error or unevenness in thickness of the non-reflective
coating would often result in 1 to 2% reflection of light at each
of the boundaries between air and lens components. Among the
reflected beams of light, surface-reflected light 71 reflected on
the surface of the quarter wave plate 53 opposite to the reflector
affects directly the measurement error of this optical system.
Except that the surface-reflected light 71 does not travel along
the measurement path 70, the surface-reflected light 71 is
rotationally polarized in the same manner as a usual measurement
path beam and then travels along the same optical path to be
detected by the light detector 9. The same thing occurs when light
passes through the quarter wave plate for the second time, causing
surface-reflected light 72 to be produced. Suppose a variation in
length of the optical path, caused by the movement of the subject
body, of the measurement path beam of each light component is
expressed to be N times the displacement of the subject body. Thus,
the beams of surface-reflected light 71, 72, which travel once
(N=2) and are mixed with a measurement path beam (N=4) that is
supposed to travel twice along the measurement path 70, are
detected by the light detector 9, causing an error in the
displacement measurement.
[0150] The same thing happens when light travels three times or
more along the measurement path as shown in FIG. 22. FIG. 22 shows
an example of an increased number of travels of light along the
measurement optical path to increase the resolution of measurement,
showing an exemplary configuration of an optical system in which a
measurement path beam travels four times along the measurement path
70. In this case, 4 to 8% of light traveling three times (N=6),
mixed with the light traveling four times (N=8), is detected by the
light detector 9, causing an error in the displacement measurement.
Similarly, light of N=4 or N=2 is also contained to cause an error
of the measurement.
[0151] Contemporary techniques cannot completely eliminate these
optical errors in the optical system. Therefore, these errors are
desirably eliminated by means of signal processing. More
specifically, the configuration as shown in FIG. 18 can be used for
this purpose. As described above, light of N=8 is mixed such as
with light of N=6 and N=4. For example, mixing of light N=2 and N=4
would cause a beat to occur due to a component wave of N=(4+2)/2=3
and a component wave of N=(4-2)/2=1. This causes an error of a long
cycle corresponding to N=1(i.e., a cycle of .lambda.). To eliminate
this error, it is desirable to generate the correction value 60 not
with the phase value 56 detected in the cycle of N=8 but with the
accumulated value 58 being employed as a variable. In addition, an
error in the shape of a triangular wave as shown in FIG. 4 means
that a number of 2N-harmonic sinusoidal components are contained in
addition to N-harmonic components. In order to eliminate these
components, it is desirable that a plurality of sinusoidal waves to
be employed as a correction value should contain 1 to 2 n harmonic
sinusoidal wave components. In this regard, the correction method
shown in Embodiment 4 is available which employs the sum of a
plurality of harmonic cyclic functions as a correction value.
Actual examples of the effects of this correction method are shown
in FIG. 23. The measurement accuracy in the range of .+-.0.6 nm as
shown in FIG. 1 is further increased up to the range of .+-.0.4 nm
by employing the sum of 1 to 2 harmonic sinusoidal and cosine waves
as the correction value 60. Configuration for correcting each of
the 1 to 2 n harmonic components makes it possible to further
increase accuracy as mentioned above when compared with the
correction of a single sinusoidal component.
[0152] On the other hand, the configuration of the automatic
tracking corrector means as shown in Embodiments 2 to 5 will
inevitably produce a time delay from the time of detecting these
cyclic error components resulted from the movement of the stage
until the components converges on an optimum correction value. More
specifically, the system is started, the stage is then move, a
signal of a .lambda. cycle component is detected, a correction
value is calculated or adjusted, and thereafter outputted is a
measurement value with increased accuracy provided by the
correction value. Accordingly, immediately after the system has
been started, the movement of the stage will first allow
measurement values containing a number of error components of a
.lambda. cycle to be outputted, and after a while, corrected
measurement values with less errors will be outputted. The delay
time is determined by the feedback time of the automatic tracking
system as described in Embodiment 5. However, when the stage is at
a standstill or moves slowly, it is impossible to distinguish an
oscillation signal resulted from interference of light from a
vibration signal caused by mechanical vibration. In this regard,
required is means for enabling or disabling the update of
correction values based on the traveling speed of the stage. This
is more specifically shown in FIG. 26. Derivator means 63 is
allowed to detect the speed of the stage based on a variation in
the accumulated value 58, and then an update enabling signal 64 is
outputted to update the correction value when the speed has
exceeded a given speed. FIG. 27 shows an exemplary configuration in
which a .lambda.-synchronous signal extractor (.lambda.-synchronous
signal extractor means) 62 detects an error of a wavelength
.lambda. cycle that is included in the measurement value 13 and
remains after correction, and feedback control is performed to
minimize the error. The update enable signal 64 controls the update
of the correction value in accordance with the speed of the subject
body (stage). This makes it possible to prevent accidental update
of the correction value at the time of low speeds. Such a
configuration allows the error of a .lambda. cycle to be
continuously suppressed even after the stage has been slowed down,
thereby making it possible to improve the measurement accuracy upon
measurement with the stage at a standstill.
[0153] As described above, the configuration of the laser
interferometer displacement measuring system according to the
present invention is characterized by comprising calculation means
for calculating a correction value for correction of the
measurement value of displacement of a subject body and an addition
mechanism for adding the correction value. The configuration is
also characterized by adding or subtracting a cyclic correction
value that is in phase with the fundamental wavelength .lambda. of
laser light. A laser interferometer displacement measuring system
with a multi-fold optical path can also have a measurement value
with increased accuracy. This configuration comprises inevitably
the calculation means for calculating and adder/subtractor means
for adding or subtracting a correction value, which follow phase
detection. An error signal included in the measurement value is
determined and the resulting error signal is added or subtracted as
a correction value, thereby providing the measurement value with
increased accuracy. The feature of the error signal makes it
possible to implement high accuracy non-contact displacement
measurement in the range of error.+-.0.4 nm or less by subtracting
1 to 2 n harmonic wave components of the wavelength .lambda. of
laser light, independent of n, using a cyclic correction value that
employs the wavelength .lambda. of laser light as a fundamental
cycle. Correction value generator means can employ a method for
generating a cyclic correction value having the cycle of the
wavelength .lambda. of laser light, with a phase value or an
accumulated value being employed as a variable. Furthermore,
available for generating correction values is an approach for
synchronously detecting a frequency signal component of the cycle
of wavelength .lambda., which is contained in a time-dependent
accumulated value. In addition, it is also possible to
automatically generate a correction value by automatic tracking so
as to minimize the signal component with the cycle of wavelength
.lambda., which is contained in the corrected signal. In this case,
since tracking is carried out so as to reduce the signal component
having a frequency component of cycle .lambda. contained in a
measurement value after the subject body has moved, the frequency
component of the measurement value varies with time.
[0154] The frequency f of the signal produced at a cycle of
wavelength .lambda. is determined by the traveling speed v of the
stage as f=Nv/.lambda. (where N is a natural number) and can be
therefore distinguished clearly from vibration signals caused by
other factors. With the averaging filter, a cut-off frequency that
is so set as to eliminate the component of frequency f would cause
signals in the frequency band greater than f to be attenuated at
the same time, thereby eliminating signals which are caused by
other oscillation factors and are originally to be detected. In
contrast, the corrector means according to the present invention is
characterized in that only an error signal caused by interference
of light can be detected by synchronous detection such as lock-in
detection and eliminated, thereby making it possible to correct the
error signal without attenuating signals which are caused by a
mechanical vibration and are originally to be detected. An actual
example is shown in FIG. 28. The displacement of the stage is
measured during the movement of the stage and the resulting signals
before and after correction are expressed in the graphs of
frequency spectrum. As shown in FIG. 28A, peaks of f=NV/.lambda.
are observed at equal intervals in the signal before correction. On
the other hand, in FIG. 28B, those peaks at equal intervals have
been eliminated in the corrected spectrum but the signals of other
high frequency band components have not been attenuated. Thus, only
such peaks of frequency f=Nv/.lambda. in phase with wavelength
.lambda. can be selectively eliminated, thereby making it possible
to carry out correction without attenuating the peaks of a
frequency component caused by other factors. In general, when
averaging applied to noise processing is carried out to attenuate a
peak of a certain frequency, signal peaks in the band greater than
the frequency are attenuated at the same time. In contrast, the
measurement value provided by the laser interferometer displacement
measuring system according to the present invention makes it
possible to selectively eliminate optical noise of cycle .lambda.,
while allowing the magnitude of vibration signals caused by
mechanical factors to remain unchanged. Use of the correction
method according to the present invention causes an error of cycle
.lambda. in the signal indicative of the amount of light received
that is detected by the light detector. Thus, the measurement of
the frequency spectrum would provide an effect of correction as in
the comparison of FIG. 28A with 28B. The feature lies in that only
those peaks corresponding to the frequency of cycle .lambda. are
selectively attenuated in the spectrum with respect to background
components. This is observed in such a manner that the peaks are
attenuated relative to the baseline (background components) of a
spectrum near the peak of a frequency f. The intensity ratio of the
peak to the baseline is to be referred to as a relative peak
intensity. That is, in increasing accuracy in the present
invention, the relative peak intensity of a peak of f=Nv/.lambda.
is selectively suppressed or attenuated. In contrast to this, in
noise processing by averaging (time averaging), a variation in gain
of frequency is gradual, and thus the peak and the baseline are
attenuated by averaging generally with the same ratio at the same
time, thereby maintaining the relative peak intensity to a constant
magnitude. This can be clearly distinguished from the elimination
of a signal of cycle .lambda. according to the present invention.
In addition, suppose the elimination of a signal of cycle .lambda.
according to the present invention is combined with the averaging.
Even in this case, since the relative peak intensity of the peak
provided by averaging to the baseline remains unchanged, a
variation in the relative peak intensity occurs only in the
elimination of a signal of cycle .lambda.. This makes it possible
to determine the availability of this approach in accordance with
relative intensity. In addition, suppose the frequency spectrum of
a measurement value is compared with a signal indicative of the
amount of light received by the light detector. The peak of
f=Nv/.lambda. of the signal indicative of the amount of light
received or a frequency component of N=n is a very signal provided
by interference of light in accordance with the principle of laser
interferometer displacement measurement and corresponds to the
component of a uniform linear motion of the stage, being outputted
as a linear increase in measurement value. Therefore, frequency
components of N.noteq.n correspond to measurement errors. In other
words, the corrector means according to the present invention is
characterized, when used, in that the peaks of N=1 to 2 n and
N.noteq.n are attenuated relative to the baseline.
[0155] However, since an error of cycle .lambda. cannot be measured
when the stage is at a standstill, the error of cycle .lambda. is
detected and corrected after the stage has been put into motion.
This allows peaks to appear in the frequency spectrum of observed
measurement values during the first movement or acceleration of the
stage. When a correction value has been set or tracking has been
performed to provide an appropriate value, these peaks are observed
to have reduced.
[0156] The laser interferometer displacement measuring system
according to the present invention is available for various types
of systems and equipment describe in Embodiment 6, which are
required for high accuracy machining, and is particularly useful
for significantly increasing the measurement or machining accuracy
of systems and equipment that employ a laser interferometer
displacement measuring system with a multi-fold optical path.
[0157] [Embodiment 8]
[0158] An Exemplary Configuration of Reducing Projection Exposure
Apparatus
[0159] FIG. 24 shows an exemplary configuration of reducing
projection exposure apparatus (a stepper for semiconductors) that
employs the laser interferometer displacement measuring system
according to the present invention. A stage for placing a wafer 81
and a photomask 82 thereon is provided on an air spring vibration
isolator 80 and protected from exterior vibration. Exposure light
84 emitted from an exposure light source 83 passes through a
shutter 85 and is then reflected on the beam bender 4 to be
introduced into the upper portion of the equipment. Subsequently,
the exposure light 84 is expanded in diameter of the beam by means
of a beam expander means 86 and passes through a lens 87 to
illuminate the photomask 82. The photomask 82 is mounted on a
photomask stage 88 and adapted to be movable. The photomask stage
88 comprises the reflector 8. The position of the photomask stage
88 is measured by three laser displacement measurement units 89
each comprising the laser interferometer displacement measuring
system, described in Embodiments 1 to 7, which can detect the
displacement and rotational error of the stage. Incidentally, an
interferometer and a light detector, which are housed inside the
laser displacement measurement unit 89, can measure the distance to
a reflector. The exposure light that has passed through the
photomask 82 is condensed to a given scale by means of a reducing
projection lens 90 and thereafter illuminated onto the wafer 81.
This causes the pattern on the photomask 82 is projected onto the
wafer 81. The exposure time is controlled by opening or closing the
shutter 85. A wafer stage 91 for placing the wafer 81 thereon
comprises the reflector 8. The positional and rotational errors of
the wafer stage 91 are measured by means of the three laser
displacement measurement units 89. It is preferable that the
photomask stage 88 and the wafer stage 91 can be moved by means of
a motor and comprises an inching mechanism for inching such as a
piezoelectric device. The accurate positional relationship between
the wafer and photomask is detected with an alignment detection
unit (alignment detection means) 92. The alignment detection means
92 comprises light-emitting and light-receiving devices and detects
alignment patterns prepared on the wafer and the photomask, thereby
making it possible to accurately measure a positional shift in
pattern in the range of accuracy of about 2 nm or less. With
employing the first alignment signal as reference, the stage is
repeatedly moved. The use of the laser interferometer displacement
measuring system that provides a high measurement accuracy will
implement an accurate alignment, thereby increasing the
superposition accuracy of the pattern to be exposed onto the wafer.
Incidentally, a configuration adapted to employ an evacuator means
93 for reducing the pressure of the entire equipment will make it
possible to use ultraviolet light of a short wavelength as an
exposure light in a vacuum, thereby implementing fine patterning.
It is also possible to prevent the effect of a variation in
refractivity of air in the measurement optical path, thereby making
it possible to further improve the superposition accuracy of a
pattern.
[0160] In addition, an integrally configured exposure apparatus
would make it possible to control the equipment integrally
including the movement of the stage. Thus, the stage can be moved
in advance at the time of initializing the equipment to detect the
error that occurs in the cycle of the wavelength of the measurement
laser light and set a correction value. In this case, it is not
always necessary to update the correction value in real time by
automatic tracking or automatic feedback. This provides an
advantage of much more simplifying the configuration of the means
for generating and updating the correction value.
[0161] Among other things, the use of the laser interferometer
displacement measuring system according to the present invention
will provides a considerable advantage in manufacturing a fine
circuit pattern, having a 0.07 .mu.m rule width or less, which
requires a measurement accuracy in the range of below 2 nm. The
wavelength of the exposure light required for the exposure of this
rule width is predicted to be 160 nm or less. The laser
interferometer displacement measuring system is particularly useful
for an exposure apparatus that employs such a short wavelength.
[0162] [Embodiment 9]
[0163] Exemplary Configuration of an Electron-beam Mask Drawing
Apparatus
[0164] FIG. 25 shows an exemplary configuration of equipment,
incorporating the laser interferometer displacement measuring
system according to the present invention, for drawing a
lithography mask (reticle) with an electron beam. The equipment
comprises the air spring vibration isolator 80 and is divided
largely into three portions of an electron-beam gun unit 100, a
stage unit 120, and a control unit 130. A high voltage is applied
between an electron-beam gun 101 and an acceleration voltage
electrode 102, thereby generating an electron beam. A beam-shaping
deflector 103 directs this electron beam to an arbitrary point on a
first beam-shaping mask 104 to shape a beam for the first time. The
electron beam that has passed through the first beam-shaping mask
104 passes through a condenser lens 105 and a blanking voltage
electrode 106, thereafter being shaped again by a second
beam-shaping mask 107. After having passed through the second
beam-shaping mask 107, the electron beam is deflected by a
positioning deflector 108 and then condensed towards a drawing
point through an objective lens 109. After having passed through a
secondary deflection electrode 110, the electron beam is
illuminated onto a master photomask 122 placed on an X-Y stage 121.
The illuminated point on the master photomask 122 is selected by
the aforementioned positioning deflector 108 and the aforementioned
secondary deflection electrode 110, and the voltage to be applied
to the aforementioned blanking voltage electrode 106 is controlled
to turn on or off the illumination of the electron beam, thereby
drawing a desired pattern on the master photomask. The control unit
130 controls these signals. The X-Y stage 121 can be moved to
select the region on the master photomask where the pattern is
drawn. The position of the X-Y stage 121 is measured accurately
with the laser displacement measurement unit 89. The laser
displacement measurement unit houses the interferometer and the
light detector (photo-detector) for measuring the displacement of
the reflector 8 on the X-Y stage 121. A motor is used to drive the
X-Y stage. The aforementioned laser displacement measurement unit
measures a control error with respect to the target position of the
X-Y stage and the voltage applied to the secondary deflection
electrode 110 is controlled for deflection correction. A wafer
height sensor 111 is used for detecting a variation in height of
the master photomask, adjusting the aforementioned objective lens,
and controlling automatically the focusing of an electron beam
illuminated onto the wafer, thereby making it possible to draw a
fine pattern with high resolution. A detection signal, measured by
the laser displacement measurement unit 89, of the light detector
is processed using the signal processing method of the laser
interferometer displacement measuring system described in
Embodiments 1 to 7, thereby making it possible to provide a high
accuracy measurement value for accurate position detection. This
makes it possible to draw a pattern on the master photomask with an
increased accuracy.
[0165] The laser interferometer displacement measuring system
according to the present invention has an outstanding effect, among
other things, on the continuous movement drawing in which an
electron beam is illuminated down to the wafer to draw a pattern
with the X-Y stage being continuously moved. When found is a
difference between the target position of the stage and the current
position measured by the laser interferometer displacement
measuring system, the aforementioned electron beam lithography
apparatus deflects the electron beam to correct the difference.
Without the approach to increasing the accuracy of measurement
values in the laser interferometer displacement measuring system
according to the present invention, an error signal having a cycle
of wavelength .lambda. of laser light is added to deflection
correction. This measurement error directly turns to be an error in
drawing a pattern. In contrast to this, with the approach to
increasing the accuracy of measurement values in the laser
interferometer displacement measuring system according to the
present invention, the measurement error of cycle .lambda. is
corrected and thereby a proper pattern is drawn. In addition,
unlike the noise processing by averaging, since it is possible to
selectively eliminate only an optical error of the laser
interferometer displacement measuring system, other signals (caused
such as by mechanical vibrations) can be detected without impairing
the frequency band. This makes it possible to minimize a control
error caused by a delay in response time. This allows for
implementing a high drawing accuracy.
[0166] In addition, an integrally configured electron beam
lithography apparatus would make it possible to control the
equipment integrally including the movement of the stage. Thus, the
stage can be moved in advance at the time of initializing the
equipment to detect the error that occurs in the cycle of the
wavelength of the measurement laser light and set a correction
value. In this case, it is not always necessary to update the
correction value in real time by automatic tracking or automatic
feedback. This provides an advantage of much more simplifying the
configuration of the means for generating and updating the
correction value.
[0167] As described above, the present invention makes it possible
to correct a measurement error corresponding to the wavelength
cycle of laser light. Thus, the present invention can provide
increased accuracy in the range of below 1 nm upon measurement of a
displacement of a sample or a target work when employed for a high
accuracy displacement measurement, instrumentation, and evaluation
technique, which requires high absolute accuracy of the order of
nanometer, or for precision and fine patterning techniques such as
semiconductor and master mask patterning.
* * * * *